This disclosure relates to the generation of electricity and steam. In particular, this disclosure relates to the generation of electricity and steam from a helium-cooled high temperature nuclear reactor by means of a closed helium Brayton cycle and a heat recovery steam generator.
Fossil fuel combustion has been identified as a significant contributor to numerous adverse environmental effects. For example, poor local air quality, regional acidification of rainfall that extends into lakes and rivers, and a global increase in atmospheric concentrations of greenhouse gases (GHG), have all been associated with the combustion of fossil fuels. In particular, increased concentrations of GHG's are a significant concern since the increased concentrations may cause a change in global temperature, thereby potentially contributing to global climatic disruption. Further, GHG's may remain in the earth's atmosphere for up to several hundred years.
One problem associated with the use of fossil fuel is that the consumption of fossil fuel correlates closely with economic and population growth. Therefore, as economies and populations continue to increase worldwide, substantial increases in the concentration of GHG's in the atmosphere are expected. A further problem associated with the use of fossil fuels is related to the inequitable geographical distribution of global petroleum resources. In particular, many industrialized economies are deficient in domestic supplies of petroleum, which forces these economies to import steadily increasing quantities of crude oil in order to meet the domestic demand for petroleum derived fuels.
Nuclear reactors do not emit appreciable pollutants or GHG's into the atmosphere and can provide energy independence to economies that are deficient in fossil fuels. The majority of today's nuclear reactors are water-cooled and generate electricity through steam generation and subsequent expansion through a steam turbine. Because of the relatively low temperature steam produced by these reactors (generally below 300° C.), the net thermal efficiency for electrical generation is relatively low (generally below 35%). An additional shortcoming of these reactors is that the steam produced is too cold for many potential industrial applications, such as hydrogen production by steam methane reforming (SMR) of natural gas or hydrogen production by solid oxide electrolysis of steam. Intermediate temperature solid oxide electrolyzer systems generally operate at temperatures of about 700 to about 900° C. Steam undergoes electrolysis in the cathode side of a solid oxide electrolyzer cell to generate hydrogen. Electrical energy is required to electrolyze the steam, so it is desirable to have a nuclear reactor system that can produce high-temperature steam as well as electrical energy.
Graphite-moderated nuclear reactors that are cooled with helium gas can achieve very high helium exit temperatures, from 700° C. to potentially 1,000° C. Many systems have been proposed for the production of electrical energy and high-temperature steam using helium-cooled reactors.
Systems have been proposed that indirectly couple a steam Rankine cycle to the primary helium coolant loop. High pressure steam is generated in a boiler heated by the helium used to cool the primary loop. The high pressure steam is partially expanded through a steam turbine to produce electrical energy. A portion of the partially expanded steam is then reheated through a second heat exchanger heated by primary loop helium. This intermediate pressure reheated steam can then be used for applications such as solid oxide electrolysis. This type of system has a risk of steam ingress into the nuclear core due to the high-pressure steam generators, where the steam can be at a higher pressure than the primary helium coolant. Steam ingress into the core is undesirable because it can corrode the graphite moderator and graphite-coated fuel, and can also cause a reactivity insertion due to the moderating effect of steam.
Other systems have been proposed that indirectly couple a Brayton topping cycle to the primary helium coolant loop and further indirectly couple a steam Rankine bottoming cycle to the Brayton cycle, in a concept known generally as an indirect combined cycle. Heat is transferred through an intermediate heat exchanger to a Brayton cycle employing a compressed gaseous working fluid, such as air or helium. This heated gas is expanded through a turbine to produce electricity. Expanded gas then passes through a heat recovery steam generator to produce steam, which can be expanded through a steam turbine for additional electrical production or alternatively can be used for industrial applications. This system does not produce steam with the required high temperature for solid oxide electrolysis, however. Furthermore, this system requires the use of a very large and expensive gas-to-gas intermediate heat exchanger.
Other systems have been proposed that directly expand the helium through a turbine to produce electricity using a direct Brayton cycle. To produce steam in addition to electricity, systems have been proposed that divert a fraction of the helium coolant exiting the nuclear core to a second loop in parallel with the Brayton cycle loop. Helium in this second parallel loop generates steam in a steam generator. Such systems have several undesirable features—they do not efficiently use the high energy available in the high-temperature helium in the second parallel loop and they use a second compressor in the second parallel loop. In addition, they use a large and expensive gas-to-gas recuperator to transfer heat from the turbine exhaust to the reactor inlet for efficient electrical generation.
It is therefore desirable to have a system that produces both electricity and low-pressure steam using a helium-cooled nuclear reactor in an economical and safe manner.